The beautiful swirls and streams surrounding the Umbrella Galaxy (NGC 4651) are the result of a dwarf galaxy being eaten up by a larger one. A study in the Monthly Notices of the Royal Astronomical Society has revealed the processes involved, and something about the unfortunate former galaxy. Besides its beauty, the Umbrella Galaxy is of particular interest because it is considered a good model for our own.

Galaxies grow by cannibalizing smaller ones, and it is common for us to see the process underway, including in our own Milky Way. Consequently its hardly news that the streams of stars and gas around NGC 4651, nicknamed the Umbrella Galaxy because the largest stream looks like the handle of a parasol, were formed through this process. However, that still left room for plenty of questions: Was one galaxy being consumed or several? How large was the meal?

In 2010 Dr David Martinez-Delgado of the University of Heidelberg examined eight isolated spiral galaxies, like our own, and found evidence of mergers in six of them. Dr Caroline Foster of the Australian Astronomical Observatory led a follow up study to explore NGC 4651 in more detail, with Martinez-Delgado as one of the co-authors. NGC 4651 was chosen because it “hosts one of the brightest and most spectacular systems of halo substructures in the local Universe,” the authors say.

“Through new techniques we have been able to measure the movements of the stars in the very distant, very faint, stellar stream in the Umbrella,” says Foster. “This allows us to reconstruct the history of the system, which we couldn’t before.” 

By tracing globular clusters that have held together despite the gravitational forces applied to them, the remnants of stellar explosions and patches of hydrogen gas the Foster and her colleagues were able to determine the movements of stars in the streams around the galaxy. They established that these streams came from a single dwarf galaxy around a fiftieth of the mass of NGC 4651 itself. This is quite similar to the ratio of the Sagittarius Dwarf Galaxy to the Milky Way. 

In the course of the study the team also tentatively identified the residual nucleus of the dwarf galaxy  and established the distance to the Umbrella Galaxy as 62 million light years. Previous estimates had been unusually dispersed.

The authors hope that their study will explain some mysteries about the halos around many galaxies, noting, “These include deficits of satellites around the Milky Way and other galaxies, and shallow dark matter cores rather than the central cusps in galaxies over a wide range of luminosities.”

^{R Jay Gabanay. This simulation based on previous work shows the distribution of remnants of the destroyed dwarf galaxy}